1. Field of the Invention
The present invention relates to a camera system and a rear conversion lens interposed between a camera body and an interchangeable lens of the camera system.
2. Related Background Art
A type of camera system has hitherto been known which uses a method of automatically focusing a subject. In this method, light being transmitted through a photographic lens is measured. There is deviation between a predetermined focal surface of the photographic lens, that is, the surface of a film, and a surface where the subject is actually formed. The amount of such deviation, called the amount of shift of an image surface (hereinafter called simply the amount of shift), is calculated. The photographic lens, which is a focusing lens, is moved in an optical axis direction in accordance with the amount of shift. The subject is thus focused automatically. In this method, the amount of movement .DELTA.X made by the focusing lens is expressed by the following equation: ##EQU1## where .DELTA.Bf is the amount of shift, and K is a factor (conversion factor) for converting the amount of movement made by the image surface, which factor causes the amount of movement .DELTA.X made by the focusing lens to correspond to the amount of shift .DELTA.Bf.
The conversion factor K is an enormous amount of data depending on the amount of shift .DELTA.Bf, and is impractical for use in an automatic focusing device. The conversion factor K is usually represented by a factor K.sub.0 for converting the amount of movement made by the image surface, which factor K.sub.0 corresponds to an extremely small amount of movement made by the focusing lens near a focal point and to an extremely small amount of shift. thus, ##EQU2##
However, when the amount of shift .DELTA.Bf increases, that is, when the difference between the conversion factors K and K.sub.0 is large, errors in the amount of movement .DELTA.X made by the focusing lens logically increase. For this reason, the focusing lens is moved beyond the focal point or approaches but does not reach it. This makes it difficult to perform a focusing operation smoothly at a high speed.
To solve such a disadvantage, Japanese Patent Laid-Open Application No. 62-170924 discloses a method of calculating the amount of movement .DELTA.X made by the focusing lens. In this method, instead of the conversion factor K, the conversion factor K.sub.0 and another factor C.sub.0 are introduced. As mentioned above, the factor K.sub.0 corresponds to an extremely small amount of movement made by the focusing lens near the focal point and to an extremely small amount of shift. The factor C.sub.0 is used for correcting the conversion factor K.sub.0 in accordance with the amount of shift .DELTA.Bf. On the basis of the amount of shift .DELTA.Bf, the amount of movement .DELTA.X made by the focusing lens is calculated from the following equation: ##EQU3##
Because of this simple operation expression, even when the amount of shift .DELTA.Bf is relatively large, the amount of movement .DELTA.X made by the focusing lens is calculated with a high degree of precision, thus solving the above problem of the focusing lens moving beyond the focal point or approaching but not reaching it. In addition, the focusing operation can be carried out smoothly at a high speed.
Equation (2) holds true when an interchangeable lens is attached directly to a camera body. However, it does not hold true any longer when a rear conversion lens is interposed between the camera body and the interchangeable lens. This is because the conversion factor K.sub.0 and the correction factor C.sub.0 are data specific to only the interchangeable lens. The automatic focusing device therefore does not function normally.
To solve this problem, an automatic focusing device disclosed in Japanese Patent Laid-Open Application No. 59-188622 has been proposed. This device calculates the amount of movement .DELTA.X made by a focusing lens in the following manner. In the case of an optical system having a rear conversion lens, the factor K.sub.0 for converting the amount of movement made by the image surface is converted to the following equation: ##EQU4## where f.sub.M = focal length of only the interchangeable lens
f.sub.MC =focal length of a composite lens including the interchangeable and rear conversion lenses PA1 .beta.c=magnification of the rear conversion lens
When the rear conversion lens is used, the amount of movement .DELTA.X made by the focusing lens, which is the interchangeable lens, is calculated from the following equation, which is converted from equation (1): ##EQU5##
However, when the equation (1") is used to calculate the amount of movement .DELTA.X made by the focusing lens, the same problem occurs as that mentioned in the conventional art, i.e., when the amount of shift .DELTA.Bf increases, errors in the amount of movement .DELTA.X made by the focusing lens logically increase. For this reason, the focusing lens is moved beyond the focal point or approaches but does not reach it. This makes it difficult to perform the focusing operation smoothly at a high speed.
Also, even if the amount of movement .DELTA.X made by the focusing lens is calculated from equation (2), to which equation (3) is applied, where only the conversion factor K.sub.0 is changed for the rear conversion lens, the conversion factor K.sub.0 is not corrected thoroughly when the rear conversion lens is used. This is because the correction factor C.sub.0 in equation (2) is a factor used only with the interchangeable lens. When the amount of shift .DELTA.Bf increases, errors in the amount of movement .DELTA.X made by the focusing lens logically increase.